As a new method for treatment of cancer, photophysicochemical diagnosis and therapy (PDT: Photodynamic Therapy) has been used. It is a method in which a certain type of porphyrin derivative is administered to a subject by, for example, intravenous injection to retain the porphyrin derivative in the target cancerous tissues in the subject, followed by laser irradiation to cause selective destruction of the cancerous tissues. The therapy utilizes the two properties of a porphyrin derivative, i.e., selectivity for cancerous tissues and photosensitivity.
The only porphyrin derivative currently used in PDT is porphymer sodium. Porphymer sodium is a mixture of products which can be prepared by treating hematoporphyrin with sulfuric acid in acetic acid and then hydrolyzing with 0.1 N sodium hydroxide, and is a 2- to 6-polymer comprising an ether and/or ester of a hematoporphyrin derivative.
However, porphymer sodium is known to cause temporary photosensitivity as an undesirable side effect when administered to man, and further, selective distribution to cancerous tissues is not sufficient for practical use, and therefore the problem of accumulation in normal tissues is present.
Under the circumstances, a patient treated with porphymer sodium is required to stay in the dark for a long period of time until it is completely excreted from the body so that normal cells are not damaged by the photosensitizing action of porphymer sodium accumulated in normal tissues. However, since porphymer sodium shows a slow excretion rate from normal tissues, it sometimes causes photosensitivity to last for more than six weeks.
In addition, PDT using porphymer sodium has a problem with transmission of the light irradiated by laser through tissues. Porphymer sodium has a longest wavelength absorption end at 630 nm and a molar absorption coefficient being as small as 3,000. Since there are many components present in a living body which prevent the transmission of light, such as oxyhemoglobin and water, the light of wavelength of 630 nm exhibits a poor transmission through tissues, which cannot sufficiently reach to deep sites, and therefore, PDT using porphymer sodium is only indicated for cancers developing in the surface layers of 5 to 10 mm depth. The wavelength which is least affected by the light absorption by the components in a living body is in a range of 650 to 750 nm, therefore , photosensitizers for PDT having the longest wavelength absorption end within such range are most desirable.
Laser devices themselves have also a problem. For example, dye lasers which are most commonly used at present have a poor stability in performance and therefore are difficult in handling in practical use. On the other hand, titanium-sapphire lasers enable to facilitate the practice of PDT considerably. However, this type of lasers are limited in the excitable wavelength to not less than 670 nm and not more than 600 nm, and therefore are not applicable to porphymer sodium which has an absorption wavelength of near 630 nm.
Recently, semiconductor lasers (670 nm), which are applicable to compounds exhibiting an absorption near 670 nm, have been developed, and quite recently OPO-YAG laser has been developed, which made it possible to cover almost all of wavelengths.
As mentioned above, photosensitizers currently used for PDT have various defects and therefore development of new agents without such defects is strongly desired. In an attempt to overcome those problems, an agent which is a single compound and exhibits its absorption in a longer wavelength region (650-800 nm) has been proposed as a second generation agent for PDT.
Examples of such second generation agent include aminolevulinic acid (ALA) which is a protoporphyrin precursor; asparthylchlorin e6 (Np e6) which is a chlorin derivative; benzoporphyrin derivative (BPD) and methatetrahydroxyphenylchlorin (m-THPC), both of which are new type of chlorin derivatives obtained by the structural conversion from hemoglobin-derived porphyrins.
In addition, the present inventors proposed chlorin derivatives and the analogues thereof, e.g., a hydroxyiminochlorinyl aspartic acid derivative (NOH-P-Asp) (Japanese Patent Application Laid-open Nos. 5-97857 and 9-124652), confirming that these compounds are useful as photosensitizers for PDT.
However, development of photosensitizers for PDT having more safety and higher therapeutic efficacy is still desired.
Therefore, it is an object of the present invention to provide a photosensitizer suitable for PDT, by finding out a porphyrin derivative which is a single component, is stable, has a higher excretion rate from normal tissues and thereby has a reduced phototoxicity, while retaining a good accumulability to cancerous tissues and, furthermore, allows the use of a titanium-sapphire laser (wavelength of not less than 670 nm and not more than 600 nm) and a semiconductor laser (wavelength of 670 nm).
The present inventors previously disclosed that when a hydroxyimino group and a residue of aspartic acid are bonded to the side chains of a chlorin, one of porphyrin derivatives, which was derivatively synthesized from hemoglobin-derived protoporphyrin dimethyl ester, a mixture of two types of position isomers represented by the following formulae (I) and (II) was obtained (Japanese Patent Application Laid-open No. 5-97857): ##STR2## wherein Asp represents a residue of aspartic acid.
The present inventors separated these two types of the position isomers from each other by means of chromatographies or recrystallization, and as a result, found that the compound of formula (I) having an imino group in the A-ring thereof has a remarkably superior accumulability to cancerous tissues as compared with the compound of formula (II) having an imino group on the B-ring thereof. In addition, it was also found that the compound of formula (I) also has a strong cell destructing effect induced by a photosensitizing reaction and a quick excretion property from normal tissues, as well as a longest wavelength absorption end at 670 nm or more.
When compound (I) was examined by albumin test [that is, a convenient test method for evaluating the affinity to cancerous tissues, in which a chlorin derivative is examined on the change in ultraviolet (UV) absorption spectrum in a mixture form with albumin and in which one of the present inventors has found a certain rule] and dancyl methionine test [that is, a convenient test method for evaluating the strength of the photoreactivity by thin layer chromatography (TLC) or high performance liquid chromatography (HPLC)] (see Japanese Patent Application Laid-open No. 5-97857), it was confirmed that the compound of formula (I) shows an excellent transferability to cancerous tissues and a strong photosensitivity.